Most of all, most of the land is in the Northern Hemisphere, and the vast majority of the big cities are in the Northern Hemisphere, too. Does this fact have a huge impact on the Earth's rotation? If so, if everybody moved to the southern hemisphere (including everything they built), would it change the rotation of the Earth?

The mass of all humans seems to be (depending on where I look for the figure) about 450 megatonnes. Hard to estimate the structures, but then as we already have a (north-biased) spread across both hemispheres, buildings and structures also, loosely estimating that all building/infrastructure item is 1000 times that of each person supported, and just smoosh "all of this is Northern" into "all of this is Southern", then compare with the mass of the Earth1 which is roughly 1010 times as heavy again (give or take an order of magnitude or two), and of which 1/1000th of the mass is water, significant amounts of which already sloshing around the capes on a daily basis... I suppose if you could make the water slosh more, it might be significant, but I suspect it'll be more of a dampner than otherwise.

You could also check the links at the start of here, for some of the maths that I decided not to go into.

1 Most of which isn't "surface", i.e. heavy mountain ranges vs (comparatively) light oceanic puddles, the weight (re)distribution of the continents being itself very slight.

If you take some surface layer of the northern hemisphere and distribute it over the southern hemisphere, all you do is shift the equator a bit relative to the continent outlines. You don't change the fact that Earth is a rotating sphere. The core is now slightly off-center, it will shift a bit afterwards, but that doesn't affect the rotation either because it is all a motion parallel to the axis of rotation.

mfb wrote:If you take some surface layer of the northern hemisphere and distribute it over the southern hemisphere, all you do is shift the equator a bit relative to the continent outlines. You don't change the fact that Earth is a rotating sphere. The core is now slightly off-center, it will shift a bit afterwards, but that doesn't affect the rotation either because it is all a motion parallel to the axis of rotation.

Question again.What if we didn't move our conditions from the Northern Hemisphere to the Southern Hemisphere, but from the north to the south, relative to the vertical axis of the Earth?see the picturep.s. and to mfb, if you are living in Seoul, why don't we use Korean instead?

I don't know about anyone else, but as far as I am concerned, you're showing the northern and southern hemispheres in that diagram as well (there may be the minor fringe disagreements arising from the disconnection of the geographic and magnetic poles, if that's your issue, but nothing significant). I've probably misunderstood or missed something, though. Try again?

The distribution of the continental plates absolutely affects the rotation, tilt, and precession of the Earth. However, the distribution of people and buildings on the earth does not. See What-If #8. Moving all the people on Earth to a single location and making them all jump at once would only change the Earth's shape by less than the width of an atom; moving all their stuff from one hemisphere to the other wouldn't make a difference.

But the question of how geographical distribution affects rotation, etc. is a really important one. In the past, when the continents were distributed differently, the length of a day was slightly different. We can track this back through ice cores and coral growth patterns.

jaap wrote:Is the biggest effect to do with moving closer/further from the poles, thereby moving mass closer/further from the axis, and changing the moment of inertia?

That's one of the effects, yes. Another effect is that the distribution of the continental plates changes tidal interactions between the moon and the Earth. Having more mass at different latitudes changes how the poles precess/wobble.

Keep in mind that the continents themselves are, quite literally, floating on a molten magma ocean. The Earth is in hydrostatic equilibrium; it maintains the same overall oblate spheroid shape. Moving any substantial mass to a different part of the planet would cause the plates in that area to sink slightly to maintain the same shape, so you'd have very little change. Of course, it would take a while; we still see isostatic rebound from melted ice sheets from the last ice age.